CAN RECRYSTALLIZATION OF MAGMATIC HORNBLENDE PRODUCE ASBESTOS MORPHOLOGIES?

The presence of naturally-occurring asbestos (NOA) has become a topic of concern for scientists, health and regulatory agencies, and citizens living in impacted areas. In southern Nevada, NOA occurs as both cross-cutting neocrystallized veins and via recrystallization of original magmatic hornblende crystals as a result of hydrothermal alteration of granitic rock. It is commonly believed that fibrous amphibole asbestos can only form through neocrystallization. This study measured the maximum length and average width of both recrystallized and neocrystallized fibers to see if the morphologies were similar. The morphology of asbestos fibers is an important characteristic that strongly affects toxicity. Fibers with a greater aspect ratio are known to have increased toxicity. Neocrystallized and recrystallized fibers from the McCullough Range, NV were identified using a petrographic microscope, extracted from polished thin sections using a motorized drill, and analyzed using the scanning electron microscope (SEM) and field emission scanning electron microscope (FESEM) with energy dispersive x-ray analysis (EDS). Neocrystallization produced fibers with an average width of 0.55 +/- 0.012 µm, an average length of 3.78 +/- 0.16 µm, and an average aspect ratio of 7.70 +/- 0.35. Recrystallization produced fibers with an average width of 0.66 +/- 0.023 µm, an average length of 6.66 +/- 0.44 µm, and an average aspect ratio of 11.54 +/- 0.76. The Mann-Whitney U-test, indicates that there is significant difference in the aspect ratios, width, and length of neocrystallized and recrystallized fibers having similar mineralogy and chemistry. Because the recrystallization process produces fibers that have greater average aspect ratios, they may be more toxic than those produced through neocrystallization. We hypothesize that the likely reason for this are increased chain width defects resulting from fluctuating conditions during recrystallization. Because hydrothermal alteration and recrystallization of primary minerals is a very common geologic process, this finding may significantly increase the number and distribution of rocks and soils that contain NOA. Therefore, many more people than we currently recognize may be unknowingly exposed to hazardous fibrous amphibole fibers.